Since the days of Edison, the incandescent light has filled many a homes and businesses with safe, convenient, and affordable illumination. Incandescent light bulbs produce light by a flow of an electric current through a filament and thereby heating the filament to a very high temperature. The filament is prevented from oxidizing or burning by encapsulating the filament within a vacuum or within an inert gas formed within a glass enclosure that allows the light to exit while preventing introduction of air/oxygen around the filament. Since the filament normally operates at extremely high temperatures, there was little need in the past to cool filament-based lighting systems.
The advances in high powered light emitting diode (LED) efficacies have exceeded incandescent and halogen light sources resulting in rapidly increasing adoption for general illumination applications. LEDs are semiconductor devices, in which, the forward biased flow of electrons across a P-N semiconductor junction produces light. LEDs are much more efficient than incandescent bulbs because more of the energy consumed by the LED is converted into light as opposed to heat (as is the case with incandescent lighting). An added benefit of LED lighting is that LEDs last much longer than incandescent lights, requiring less frequent replacement. The long life offsets an initially higher cost to produce LEDs. Typically, LEDs have lifetimes of 50,000 hours or more when operated at around 25° C.
LED light output (or flux) is measured in lumens. Led light output and reliability are dependent upon temperature, a common characteristic for all LEDs. As LED case temperatures and corresponding junction temperatures increase, light output decreases and reliability typically decreases. Therefore, proper thermal management of the LEDs is critical to minimize the reduction in light output and maintain the expected reliability of the LEDs. Furthermore, because LEDs are semiconductors, they have a limited operating temperature range and will fail or have limited life if operated above that temperature.
For many applications, LEDs fit in well, replacing incandescent equivalents without significant problems. Applications where there is sufficient air flow often provide sufficient cooling to properly operate LED based incandescent replacement bulbs because the ambient room air temperature is typically what is comfortable to people, between 60° F. to 80° F. Applications such as in a table lamp provide a reasonable ambient room air temperature for operation of an LED-replacement bulb.
There are many applications where the ambient temperature is much higher than 60° F. to 80° F., creating problems with cooling the LEDs. One such example is in overhead recessed lighting (e.g. “Can Lights” or “Top Hats”). Such lighting is often recessed above a ceiling with little or no air circulation from the room below. In such cases, the heat sinks used to cool the LEDs are often located within the un-cooled space between the ceilings and next floor of a building or directly below an attic and often covered with insulation. In such cases, the air space around the heat sink is dead air space with typical ambient temperatures that often exceed 60° C. (140 F). These heat issues were less of a problem with incandescent light bulbs that are designed to operate in such high temperatures. However, these heat issues are critical issues for LED lighting.
To permit operation of LED lighting in recessed lighting, manufacturers have resorted to including extra-large heat sinks to channel heat away from the LEDs. Such heat sinks help, but due to the typical dead air space temperatures, these heat sinks are not sufficient solutions for many applications. Furthermore, there are limitations on the size of such heat sinks due to the typical space above the ceiling and installation spaces such as the hole size through which the recessed lighting must pass during installation.
Typical LED recessed lighting applications resort to large and heavy heat sinks to transfer heat from the LED junctions. Numerous light fixture applications exist where the heat sink is designed to be located in relatively high ambient temperatures of the dead space which are greater than room temperatures. Room temperatures are typically in the range of 18° C. to 26° C., but dead air spaces typically reach temperatures of anywhere from 40° C. to 60° C. Flow of heat from the LED junctions, through the heat sinks, and out to the surrounding air depends upon the temperature differential between the junction temperature and the temperature of the surrounding air. For example, if the junction temperature is 60° C. and the surrounding air temperature is also 60° C., no heat will flow and no heat will be dissipated.
LED recessed down lights are often enclosed within a can enclosure which is in turn installed in a ceiling in commercial buildings. In such cases, dead air space exists between the next floor and a dropped ceiling constraining the heat flow from heat sinks. Similarly, in residential applications, LED recessed down lights are often installed in ceilings below an attic. In these attic locations, an insulation layer often surrounds the recessed down lights, further reducing the heat flow from LED loads to the air above the insulation layer.
What is needed is a LED heat sink system that will dissipate sufficient heat such that the LEDs will operate within their specified temperature ranges in ceiling lighting systems.